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The best sanitary engineers in the solar system are now relegated to "snag finding" in the lower levels of Space Station Alpha. (the largest artificial satellite in orbit around the Earth in the year 3769)

Welding Robot X-5 "Bert" and his mate Jimmy are carrying out a routine inspection of a waste outlet pipe that appears to have gone missing. Jimmy is working on snag-finding duties to raise money to repair the broken bionic rivet gun attached to his arm. When it is fixed they can return to the top executive levels, working for better credits.

All of a sudden, Bert's antigrav goes offline and he falls down a thruster duct. There he meets the Tenth Doctor. All suited up, the Doctor is impressed to find a Technomatic — "the best construction robots ever invented". Introductions are cut short when Bert's connections seem to fail and he accidentally zooms off, dragging the Doctor behind with him. When he comes to rest they are rejoined by Jimmy. The Doctor diagnoses Bert's erratic behaviour, attributing it to a secret signal on an unauthorised wavelength emanating from Block 17.

Taking Bert's receptors offline, Bert and Jimmy accompany the Doctor to investigate the source of the signal. Behind closed doors in the waste containment block, they find armed and unfriendly androids. When disturbed, they activated and threaten to "terminate the humanoids".

Escaping down the missing waste outlet pipe the Doctor, Jimmy and Bert come face to face with the Klytode, who unlike its guard androids, is ready. "Ready to destroy the planet Earth"!

The Klytode is armed with a cobalt bomb, "the most destructive weapon in the solar system". The bomb is primed to explode and blast Space Station Alpha out of orbit and crash into the Earth.

Bert leaps into action, snatches the bomb from the Klytode and carries Jimmy and the Doctor off — pursued by the androids. Deep within the Space Station, they are unable to call for help. The Klytode is also unable to send a low-frequency delta waves signal to detonate the bomb.

The Klytode doesn't give up and catches up with them on the higher levels where the Klytode takes back possession of the bomb. The Klytode (a gestalt being — a powerful mind with many bodies — plans to use the bomb to crash into the Earth, setting off a chain of events that will change the climate of the planet and see it reborn as a Klytode paradise. The Klytode brethren will arrive and drain the planet. They will turn it into an exhausted husk like its own homeworld on the edge of the Aktren galaxy and other planets that the Klytode have inhabited.

However, the Doctor has a plan. On his command, Bert sends a jamming signal on the same unauthorised frequency used to scramble his own controls. The androids are rendered useless and the Klytode weakened. Snatching back the bomb, the Doctor quickly repairs to Jimmy's bionic rivet cannon and shoots the bomb into space. Help soon arrives and the Klytode is arrested and sent back to the Artren galaxy. Bert and Jimmy (now with a repaired rivet cannon), are destined to become executive waste engineers once again.